File:Eclogite (Late Silurian to Early Devonian, ~400-423 Ma; Almenningen, Vestland, Norway) 1.jpg

English: Eclogite from the Middle Paleozoic of Norway.

This is eclogite, an attractive, uncommon, crystalline-textured, very high-grade metamorphic rock. The reddish mineral is garnet (pyrope or almandine) and the green is omphacite pyroxene. Also present is whitish-gray quartz, bluish kyanite, minor mica, tiny rutile crystals, and minor blackish-colored amphibole.

Eclogites form in the mantle, but occur in few places at the Earth’s surface. These rocks have the same chemistry, but different mineralogy, as basalts and gabbros (= oceanic crustal rocks). Eclogites form by very high grade metamorphism of oceanic crust at mantle depths along subduction zones. Uplift of eclogites back to the surface usually involves some retrograde metamorphism and the formation of new minerals, resulting in retrograde eclogites.

The eclogite seen here dates to the Late Silurian to Early Devonian (~400-423 Ma), when Proterozoic-aged protolith rocks were subjected to very high-grade burial metamorphism. This took place during the Scandian Orogeny, a mountain building event that involved oceanic lithosphere adjacent to ancient Baltica (= the ancient Scandinavian continent) being subducted beneath Laurentia (= the ancient North American-Greenland continent). Eclogites were brought back to upper crustal levels during the Devonian portion of the Caledonian Orogeny (~383-404 Ma).

Locality: Almenningen, Vågsøy, Kinn, Vestland, southwestern Norway